Potato plants are infected by many viruses of economic importance. Potato field surveys often find plants infected with more than one virus that exhibit disease symptoms which are more severe than plants infected with only one virus. Potato Virus X (PVX) is present in many potato fields generally causing mild symptoms and has been referred to as the "healthy potato virus." The presence of mild strains of PVX may protect potato plants from infection with severe PVX strains.
Plants doubly infected with PVX and Potato Virus Y (PVY) may produce severe disease symptoms with high PVX titers (Rochow and Ross, 1974). PVX titers have been shown to increase as much as ten fold in the presence of PVY, with mild strains of PVX increasing more than severe strains. Apparently, the increase in PVX titers in dual viral infections is due to increased synthesis of PVX per cell as opposed to an increase in the number of infected cells (Goodman and Ross, 1974). Tobacco Mosaic Virus (TMV) and/or Cucumber Mosaic Virus (CMV) dual infection with PVX in tobacco plants showed a synergistic increase of disease symptoms but only TMV/PVX and PVY/PVX infections resulted in increased PVX titers (Close, 1964). PVX must be present in the plant before inoculation with TMV or PVY to exhibit the synergistic activity. The PVX/PVY synergism does not occur in plants systemically infected first with PVY (Rochow and Ross, 1974). It has been suggested that the PVX/PVY synergism is caused by PVY interfering with a factor produced by PVX which limits PVX replication (Damirdagh and Ross, 1967).
Genetically engineered resistance to virus infection by expression of viral coat protein in plants has been demonstrated for Tobacco Mosaic Virus (Powell-Abel, et al., 1986), Alfalfa Mosaic Virus (Tumer, et al., 1987), Cucumber Mosaic Virus (Cuozzo, et al., 1988), Potato Virus X (Hemenway, et al., 1988), and Tobacco Streak Virus (van Dun, et al., 1988). However, the pronounced synergistic effect of PVX/PVY infections in plants such as tobacco and more particularly potato renders it unpredictable whether the coat protein expression route would be efficacious in this situation.